Human error in preparing chromatographic eluent solutions, as well as contamination of these solutions on storage or during other periods of disuse, limits the formation of eluents of precise composition and the reproducibility of eluent composition from analysis to analysis, adversely affecting the reproducibility of chromatographic analyses. An important and well known example is the preparation of hydroxide eluents for ion chromatography (IC) where CO2 intrusion into the eluent results in carbonate contamination of the hydroxide eluent. The longer an eluent sits idle exposed to ambient air, the more the carbonate concentration increases. Thus, devices and methods for reproducible formation of eluents on an “as needed” basis are needed in liquid and ion chromatography.
A different area of concern is that eluent components essential in the separation process may be detrimental for detection. For example, it is challenging to separate amino acids, peptides, and proteins without any ionic constituents in the eluent. However, when using electrospray ionization mass spectrometry (ESI-MS), the most commonly used detection method today for such separations, the ionic constituents significantly interfere with the analyte measurement.
An electrodialytic membrane based eluent generator was invented to produce high purity reproducible hydroxide eluents on-line, on-demand (Strong, et al. Anal. Chem. 1991, 63 (5):480-486). U.S. Pat. No. 5,045,204 discloses an exemplary eluent generator. The device generates a high purity aqueous stream with selected ionic species—either cation (e.g. sodium) or anion (e.g. sulfate) suitable for use as a chromatography eluent. In one embodiment, the eluent generator includes a source channel and a product channel separated by a permselective ion exchange membrane including exchangeable ions of the same charge as the selected ionic species. The membrane allows passage of ions of the same charge as the ionic species but is resistant to transmembrane passage of ions of opposite charge. An electrical potential is applied between the source channel and product channel.
Other exemplary eluent generators are described in U.S. Pat. No. 6,036,921 and U.S. Pat. No. 6,225,129. All above devices are electrolytic devices disclosed as being useful to generate high purity acid and base solutions by using water as the carrier. Using these devices, high purity, contaminant-free acid or base solutions are automatically generated on-line for use as eluents in chromatographic separations. These devices simplify gradient separations that can now be performed using electrical current gradients with minimal delay instead of using a conventional mechanical gradient pump.
Another exemplary eluent generator is described in U.S. Pat. No. 8,647,576. An acid or base is generated in an aqueous solution by the steps of: (a) providing a source of first ions adjacent an aqueous liquid in a first acid or base generation zone, separated by a first barrier (e.g., anion exchange membrane) substantially preventing liquid flow and transporting ions only of the same charge as said first ions, (b) providing a source of second ions of opposite charge adjacent an aqueous liquid in a second acid or base generation zone, separated by a second barrier transporting ions only of the same charge as the second ions, and (c) transporting ions across the first barrier by applying an electrical potential through said first and second zones to generate an acid-containing aqueous solution in one of said first or second zones and a base-containing aqueous solution in the other one which may be combined to form a salt.
In electrolytic eluent generators, concentration may be controlled by simply altering the current applied to the device. In chromatographic separations, it is often necessary to generate gradients where the pH and ionic strength are varied independently or together, with or without an organic solvent included in the gradient. The complexity of the controlled generation of such eluent gradients is not only increased by the number of individual solutions/fluids that must be made/handled, the complexity of such preparation increases with a proportionate toll on reproducibility. The rise of ultrahigh performance chromatography (UPLC), smaller particles sizes, and shorter columns has also seen the need to use high pressure mixing and multiple pumps in order to reduce the gradient delay time. Eluent generators on the high pressure side have the advantage of lower delay times and obviate the need for a mixing chamber. In addition, the widespread use of electrospray ionization mass-spectrometry (MS) has made the demand for very high purity volatile buffers a necessity.
Recently, it was shown that gas permeable membranes can be used for the addition of basic gases into an eluent stream to raise the pH for the conductometric detection of weak acids (Liao, et al., Anal. Chem. 2016, 88 (4):2198-2204).
The use of neutral gases and gases derived from organic solvents (e.g., MeOH, CH3CN), ammonia and volatile bases and volatile acids to alter the composition and characteristics of an eluent stream in an eluent generator provides a different and promising addition to the current range of electrolytic eluent generators. If such devices could be configured to produce eluents of predictable and reproducible composition and characteristics in isocratic and gradient formats, this would represent a significant improvement in the field of chromatography and generating eluents for chromatography. The present invention provides such a device, systems incorporating this device and methods of using the device and systems in chromatographic separations.